User Authentication question

[CavemanUK]

Hi guys,

For a few years I've been playing around with a php driven website that manages the customers in our small business. Its very low usage and more of a fun project than anything... recently ive been looking at making it more mobile and more html5/css3 specific etc.

Anyway, I was thinking about the user authentication. Currently there are 3 users that are stored in a mysql database and validated in the obvious way.

I wanted to improve the security and stability of this side of things and was wondering if this is something i can use something like facebook or openid for? basically i want to allow the same 3 users access but nobody else. it seems like facebooks authentication would certainly ensure its the right person logging in.

does this make any kind of sense or is there a better way?